A common method is to subtract your age from 220. That will give you your max heart rate. (In my case, 220-39 = MHR of 181). Once you have determined that, come up with some targets: I use 65% for fat-burning workouts, which comes out to 118, and 80% for cardio workouts, which is 145. I often push it to 90%, which for me is 162, and sometimes I'll go higher.

Choose a target based on the goal of your working out. You should work toward keeping your heart rate in its target zone for at least 20 minutes.

There is a lot of excellent exercise advice in the exercise forum - Trainer Dan, Fern and Nat have the answers!

Best of luck,